1. Field of the Invention:
The present invention relates to a color cathode ray tube apparatus which comprises an in-line type color cathode ray tube and an associated focusing means.
2. Description of the Prior Art:
In an in-line type cathode ray tube (hereinafter abbreviated as CRT) which comprises three electron beam outlets disposed on a horizontal straight line, using a saddle type or a toroidal type deflection yoke as a beam deflection means, the horizontal deflection magnetic field is distorted into a pincushion shape and the vertical deflection magnetic field is distorted into a barrel shape. As a result, the obtained self-convergence effect can sharply simplify the component of the focusing system.
On the contrary, as shown in FIG. 1, at the phosphor screen 1, especially the fringe parts of the phosphor screen, here the beam spot 2 which is distorted to be non-circular by the deflection induced beam distortion, therefore resolution is deteriorated. The beam spot 2 comprise a horizontally elliptical high luminance core portion 3 and its attendant low luminance haze portion 4. Such lowering of the resolution by the deflection induced beam distortion can be alleviated by applying a shorter distance between the main focus lens and the cathode of the electron gun to decrease the diameter of the electron beam passing through the main focus lens of the electron gun and the deflection field or only by decreasing the diameter of the electron beam with a stronger pre-focus lens. These measures increase the magnification of the lens system of the electron gun, and as the result the diameter of the beam spot around the centre part of the screen becomes undesirably large.
As shown in FIG. 2, the optimum focus voltage for the horizontal diameter of the beam spot is immutable at any point of the phosphor screen, while the optimum voltage for the diameter becomes higher as the spot goes to the fringe part of the phosphor screen (especially at E and NE in FIG. 1)
Accordingly, when driving at the optimum focus voltage (which is 6 KV in FIG. 2) for the horizontal diameter, the beam spot at the fringe part of the phosphor screen becomes over-focussed in the vertical direction and the above-mentioned vertical haze is produced resulting in the deterioration of the resolution.
As shown in FIG. 3, the resolution in the fringe parts of the phosphor screen can be enhanced without producing such vertical haze by sacrificing resolution at the center part of the phosphor screen through the lowering of the vertical focusing voltage than the horizontal optimum focusing voltage although resolution is lowered a little at the center part.